Circuit interrupting means for a high voltage direct-current circuit with means for reducing the severity of the recovery voltage



3,252,050 GE DIRECT-C URRENT 3 Sheets-Sheet 1 //v VENTOR. THOMAS H LEE,

ATTORNEY y 1966 T. H. LEE

CIRCUIT INTERRUPTING MEANS FOR A HIGH VOLTA CIRCUIT WITH MEANS FOR REDUCING THE SEVERITY 7 OF THE RECOVERY VOLTAGE Filed April 7, 1964 A H NQN w/ /H .QN la a R R iv w \M H v Q l x May 17, 1966 T. H. LEE 3,252,050 CIRCUIT INTERRUPTING MEANS FOR A HIGH VOLTAGE DIRECT-CURRENT CIRCUIT WITH MEANS FOR REDUCING THE SEVERITY OF THE RECOVERY VOLTAGE Filed April '7, 1964 3 Sheets-Sheet z THOMAS H. LEE,

May 17, 1966 T. H. LEE 3,252,050

CIRCUIT INTERRUPTING MEANS FOR A HIGH VOLTAGE DIRECT-CURRENT CIRCUIT WITH MEANS FOR REDUCING THE SEVERITY OF THE RECOVERY VOLTAGE Filed April '7, 1964 3 Sheets-Sheet 5 :3 "/07 PUL 6 E SOUR CE //VVE/V7'0/?.' THOMAS H LE5,

ATTORNEY United States Patent 5 Claims. (Cl. 317-11) This invention relates to means for interrupting a high voltage direct-current circuit and relates, more particularly, to circuit interrupting means of the'type in which a current Zero is created by forcing a locally-controlled current through a circuit interrupting unit in opposition to the load or fault current.

In the particular circuit interrupting means that I am concerned with, a circuit interrupting unit, hereinafter referred to as an interrupter, is connected in series with a high voltage direct-current line that supplies current through the interrupter to a load. When the circuit is to be interrupted, the normally-closed contacts of the interrupter are separated to establish an arc across the gap developed between the contacts. Connected across this gap is a normally-open quenching circuit that inclues a pre-charged commutating capacitor. When the abovedescribed are is established across the interrupter gap, the capacitor is discharged through the quenching circuit and the interrupter in a direction in opposition to the load current. This discharge current is higher than the maximum load current being interrupted, and thus the resultant current through the interrupter is quickly forced to zero. By the time the current zero point is reached, the gap between the contacts has attained a substantial length. If this gap is able to withstand the recovery voltage that rapidly builds up thereacross at current zero, then circuit interruption is successfully completed.

The character of the recovery voltage is determined to a large extent by the values of capacitance, inductance and resistance then present in a circuit that extends between the terminals of the D.-C. source through the D.-C. line, the commutating capacitor, and any load remaining in the power line. This circuit is usually an oscillatory circuit that includes the series combination of the system inductance and the -commutating capacitance. Oscillation between this inductance and capacitance commences as soon as the current path through the interrupter unit is interrupted, as above described, and this oscillation constitutes a major component of the recovery voltage.

An object of my invention is to reduce the frequency and peak value of the voltage developed by this oscilla tion, so as to improve the interrupters chances for successfully withstanding this voltage without breaking down.

One way of attaining this object is to increase the size of the commutat-ing capacitor, but this approach is limited because economic considerations dictate that this capacitor be as small as possible.

Accordingly, another object of my invention is to reduce the frequency and peak value of the voltage developed by this oscillation without increasing the size of the commutating capacitor or without introducing additional costly capacitance in parallel with the commutating capacitor.

In carrying out my invention in one form, I provide a DIRECT-CURRENT CIRCUET WITH tially of insulating material, and a 3,252,056 Fatented lViay 17, 1966 normally-open damping circuit that is connected across the usual smoothing reactor of the DC. system, which reactor constitutes a substantial portion of the system inductance. The damping circuit comprises the series combination of a resistor and a normally-open gap 'device. The gap device is caused to break down soon after the current through the interrupter is forced to zero, and this effectively closes the damping circuit, thus connecting the resistor across the smoothing reactor. The presence of this resistor materially lowers the frequency and eak value of the recovery voltage appearing across the interrupter gap when the current therethrough is forced to zero, thus materially improving the interrupters chances for withstanding the recovery voltage.

In accordance with another feature of my invention, I use for the interrupter a vacuum-type circuit interrupter. A circuit interrupter of this particular type can recover its dielectric strength at an extremely high rate when the above-described current zero is produced. As will be eX- plained later, this enables the commutating capacitor to be relatively small, and this in turn enables me to use a resistor in series with the gap device that has a relatively high resistance. With a relatively high resistance in this circuit, the energy dissipation requirements of the gap device can be kept low, and this permits its breakdown voltage to be maintained substantially constant despite repeated arcing experiences.

For a better understanding of my invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic showing of a circuit interrupting arrangement embodying one form of my invention. The circuit interrupting arrangement is shown in its normallyclosed position.

FIG. 2 is a schematic showing of the circuit interrupting arrangement of FIG. 1 shown during an interrupting operation.

FIG. 3 is a schematic showing of a circuit interrupting arrangement embodying a modified form of my invention.

, Referring now to FIG. 1, there a high voltage D.-C. circuit comprising a source 12, a load 14, and a power line 16 for delivering power to the load from the source. It will be assumed that the normal load current flows in the direction indicated by the arrow 17, returning to the source through a return conductor 19. The source 12 is schematically depicted as comprising a transformer 20 and a rectifier 21 connected in series with the secondary winding of the transformer. Connected in the power line 16 and in series with the source 12 and the load 14 is the usual smoothing reactor 18 which acts to smooth the current output from the source.

For controlling the flow of current to the load 14, a circuit interrupter 25 is connected in the power line 16 in series with the load 14 and the smoothing reactor 18. In a preferred embodiment of my invention, the circuit interrupter 25 is a vacuum-type circuit interrupter. As such, it comprises a highly evacuated envelope 26 parpair of relatively movable contacts 27 and 28 disposed within the evacuated envelope 26. The upper contact 27 is a stationary contact, and the lower contact 28 is a movable contact that projects through the lower end of the envelope 26. A suitable bellows 29 sealed at its respective opposite ends to the lower contact 28 and the envelope 26 permits the is schematically shown plained in greater detail.

' inside the envelope.

is the general approach used lower contacts'to move vertically without impairing the vacuum inside the evacuated envelope 26. The lower contact 28 is releasably held in its closed position of FIG. 1 by a suitable latch 30 and is biased in a downward or opening direction by a suitable opening spring 32. When the latch 30 is released, thespring 32 drives the movablecontact 28 downwardly to produce a gap 33 between the contacts, as shown in FIG. 2. This contactseparation establishes an are 34 across the gap that is quickly extinguished and prevented from reigniting, thus interrupting the circuit, all in a manner soon to be ex- For condensing the metallic vapors generated by the arc and for protecting the insulation of-the envelope 26 from these vapors, a suitable metallic shield 31 of tubular configuration is provided In view of their role in initiating the arc, the latch 30 and opening spring 32 may be thought of as portions of arc-initiating means used! for initiating are 34.

The vacuum circuit interrupter 25 can be of a suitable I conventional type and is therefore shown in schematic form only. Examples of vacuum type circuit interrupters suitable for this application are shown in more detail and are claimed in US. Patents 2,949,520, Schneider, and 3,089,936, Smith, both assigned to the assignee of the present invention.

It is considerably more difficult tointerrupt direct current than alternating current because direct current contains no naturally-occurring current zeros. With alternating currents, current zeros occur naturally, and to interrupt such currents, it is only necessary to prevent reignition of the arc after a natural current Zero. But with direct current, it is necessary first to force the current to zero and then to prevent arc-reignition.

One way of forcing the current to zero is by forcing a locally-controlled current through the interrupter in opposition to the load current flowing therethrough. This in the illustrated interrupting arrangement, where the opposing current is derived from a commutating capacitor 35 that is precharged with the polarity shown in FIG. 1. This commutating capacitor 35 is located in an arc-quenching circuit 36 that is connected across the contacts 27, 28 of the interrupter v 25. The arc-quenching circuit 36 is normally maintained in an open position by means of a normally-open switch 38 connected in series with the commutating capacitor 35. The illustrated normally-open switch 38 is a vacuum-type circuit interrupter similar in construction to l the other interrupter 25. Accordingly, those parts of the switch 38 that correspond to similar parts of the interrupter 25 have been assigned corresponding reference numerials with the suffix a.

When .the normally-open switch 38 is operated from its open position shown in FIG. 1 to a closed position shown in FIG. 2, commutating capacitor 35 discharges in the direction of arrows 39 through the quenchingcircuit 36, forcing current through the interrupter 25 in a direction opposed to the normal load current therethrough. The opposing current I can be expressed as follows:

I =Vc sin where V is the voltage across the capacitor 35 before the switch 38 is closed; C is the capacitance of capacitor 35, and L is the inductance in the circuit comprising quenching circuit 36 and the interrupter 25, and t is the time measuredfrom the instant that switch 38 is closed. To interrupt a current I flowing through the interrupter 25 in the direction of arrow 17, it is necessary that the opposing current I be considerably higher than the maximum value of I Accordingly, with a given size commutating capacitor 35, the precharge voltage V on the capacitor is made high enough to provide an opposing current I considerably higher than the maximum value of I that it is desired to interrupt.

In a preferred form of my invention, circuit interruption is effected by first separating the contacts 27 and 28 to draw an arc therebetween, as shown in FIG. 2. At a predetermined instant thereafter, the contacts 28a, 27a of switch 38 are driven into engagement to complete the quenching circuit 36, as shown in FIG. 2. This forces a reverse current I through the quenching circuit in the direction of arrows 39 and through the arc in interrupter in the direction opposed to the load current 1;, flowing therethrough. Since I is higher than I the resultant current flow through the arc is quickly driven to zero, thus extinguishing the arc.

Interruption is successfully completed, however, only if the gap 33 that is then present between the contacts 27, 28 can successfully Withstand the recovery voltage 34 in the interrupter 25. Thus, after this are is extinguished, there is still a large amount of stored energy remaining in the inductance to produce continued current flow, which now takes place through the quenching circuit 36 containing commutating capacitor 35. This sets up an oscillation, and such oscillation transfers energy back and forth between the inductance 18 and the capacitor 35.

The transfer of energy from the inductance 18 to the capacitor will create a component of voltage across the capacitor, and hence across the inter-contact gap 33, that varies as a function of where I is the current in the inductance 18 when the arc is extinguished and L is the inductance and C the capacitance of a circuit that extends through reactor 18, power line 16, quenchingcircuit 36, any part of the load 14 that is still present, and the return conductor 19. By connecting a resistor across the inductance 18 during this interval, I can very substantially reduce the peak value and frequency of this voltage. Such a resistor is shown at connected in series with a gap device 52. The series combination of resistor 50 and gap device 52 forms a damping circuit that is connected across the terminals of the smoothing reactor 18. Under normal conditions, the gap device 52, being non-conducting,

maintains the damping circuit 50, 52 open and thus prevents current from flowing through the resistor 50. But when the gap deviceSZ breaks down, the resistor 50 is connected across the smoothing reactor 18, thus substantially reducing the peak value and frequency of the voltage appearing across the capacitor and interrupter 25 as a result of this oscillation. By reducing the peak value and frequency of this voltage that results from this oscillation, the likelihood that the system insulation and the inter-contact gap 33 will be able to successfully withstand the recovery voltage is appreciably increased. The gap device 52 is set to break down when the voltage thereacross reaches a predetermined value which corresponds to a voltage between lines 16 and 19 of to 250% of normal system voltage.

In a preferred form of my invention, the resistance of resistor 50 in ohms is between where L is the inductance of smoothing reactor 18 in henries and C is the capacitance of capacitor 35 in farads.

lation are to increase the size of capacitor 35 or to connect an additional capacitor or a resistor-capacitor combination in parallel with the capacitor 35. But for high voltage applications none of these approaches is very desirable, since they would greatly increase the cost of the interrupting arrangement in view of the large amount of expensive capacitance required. A special advantage of my interrupting arrangement is that it enables me to reduce the frequency and peak amplitude of the voltage resulting from this oscillation without necessitating either of these expensive additions to the capacitance 35.

In addition to the component of recovery voltage represented by the above-described oscillation there is another component that develops when current ceases flowing through the interrupter 25. This additional component results from the action of the system voltage, i.e., the voltage across source 12, in charging the commutating capacitor 35 when current ceases flowing through interrupter 25. This charging action takes place through the circuit 16, 36, 19 and tends to reverse the polarity of the commutating capacitor 35 from that shown in FIG. 1. Ultimately the capacitor 35 will be charged to the system voltage and there willthen be no more current flowing from the power line 16 into the quenching circuit 36. When this occurs, the switch 38 is suitably returned from its closed position of FIG. 2 to its open circuit position of FIG. 1. Since no current is then flowing through the quenching circuit 36, no arcing occurs between contacts 27a, 28a when they are then separated.

During this charging of the capacitor 35 by the system voltage, the resistor 50 is effectively connected in series with the line 16 inasmuch as the gap device 52 is then conducting. The presence of this resistor introduces considerable damping into the charging circuit and thus limits the overshoot in voltage across capacitor 35 resulting from this charging action.

capacitor 35 corresponds-to the recovery voltage appearing across interrupter 25, it will bev apparent that the resistor 50 has also lessened the severity of the voltage that this latter component imposes upon the gap 33 of the interrupter 25 and upon the system insulation.

When the capacitor 35 has been charged to line voltage as above described, there is no longer any current flow through the power line 16. This being the case, 'the are at gap device 52 is extinguished, and the resistor '50 is thuseffectively removed from the power line 16.

A vacuum-type circuit interrupter is the ideal type of interrupter for use as the main interrupter element in this general type of interrupting arrangement. In this respect, it is quiet and simple, has long contact life, and is able to interrupt consistently at the first currentzero. Moreover, the contact gap required by a vacuum-type lnterrupter for a given voltage is exceptionally small,

.thus permitting shorter operating times and a simpler operating mechanism. But perhaps the greatest advantage of the vacuum-type interrupter is its extremely rapid recovery of dielectric strength following a ra id decrease of current prior to current zero (such as occurs when the reverse current drives the load current to zero, as above described). As an example of this extremely rapid recovery of dielectric strength, a single-break vacuum interrupter of the type disclosed herein has been found capable of withstanding recovery voltages up to 20 kv. at a rate of 3300 volts per microsecond following a rapid decrease of current prior to current zero (for example, a decrease at a rate of 190 amperes per microsecond).

The higher is the rate of dielectric recovery following this current zero, the higher is .the permissible rate of rise of the recovery voltage and, hence, the smaller may be the capacitor 35 inasmuch as the rate of rise of the recovery voltage varies inversely with the size of the Since this voltage across device to withstand the recovery voltage transient.

6 capacitor. Not only is it advantageous from the viewpoint of reducing the capacitor cost to make the capacitor as small as possible, but also it is highly advantageous from the viewpoint of providing a gap device 52 capable of consistently performing in the desired manner. In this latter respect the smaller the capacitor 35, the higher may be the resistance of resistor 50 since the resistance has to be kept small compared to the where L is the inductance and C is the capacitance of circuit 16, 36, 19. By using a relatively high resistance for resistor 50, a relatively large percentage of the total energy stored in the inductance 18 that is dissipated in the damping circuit 50, 52 is dissipated in the resistor 50 in stead of in the gap device 52. By limiting the energy dissipation in the gap device 52, the rate at which its electrodes are consumed by arcing is held to a relatively low value and hence its breakdown voltage can be held relatively constant despite repeated arcing experiences. It will therefore be apparent that I utilize the extremely high dielectric recovery rate of a vacuum-type interrupter to reduce the energy dissipation requirements of the gap device 52, thus preserving its dielectric breakdown properties despite repeated arcing experiences.

A problem that arises from reducing the size of the commutating capacitor is that the rate of decrease of the current just prior to current zero becomes higher as the capacitor is made smaller. The higher this rate of decrease, the more diflicult it is for an interrupting But, as pointed out hereinabove, a vacuum-type interrupter has an exceptional ability to recover its dielectric strength despite a high rate of current decrease prior to current zero. For this additional reason, then, the use of a vacuum-type interrupter permits a relatively small commutating capacitor 35 as compared to that neeeded with other types of interrupters.

Although I have shown a single-break vacuum interrupter being used as the main interrupting element 25, it is to be understood that for high voltage applications, a number of these units must be connected in series to withstand the high voltages involved. My invention thereclosing switch 38 to its closed position.

fore comprehends an arrangement in which the main interrupter comprises a plurality of interrupting units con- .nected in series for substantially simultaneous opening and for substantially simultaneous closing.

The control circuits for operating the two interrupters 25 and 28 in the desired sequence may be of any suitable conventional type and I have therefore shown them in a highly simplified schematic form. Referring to FIG. 1, the control circuit for the latch 30 of interrupter 25 comprises a conductor 60 extending from the positive to the negative terminals of a control voltage sourcethrough the normally-open contacts 62a of an overcurrent-responsive relay 62 and the coil of a tripping solenoid 64, which controls the latch 30. When the currentin line 16 exceeds a predetermined value, the relay 62 picks up, closing its contacts 62a to complete an energizing circuit for the solenoid 64. The solenoid 64 responds by tripping the latch 30 to cause opening of the interrupter 25.

When the movable contact 28 of the interrupter 25 has reached a predetermined point in its opening stroke, it completes a controlcircuit 70 that causes the closing switch 38 to be operated to closed position. This control circuit 70 comprises a normally-open switch 72 which is closed by a cam on the movable contact 28 when the movable contact reaches a predetermined point in its opening stroke. This completes an energizing circuit for a closing solenoid 76, which responds by operating the After a prede termined period of time sufiicient to enable the current in quenching circuit 36 to be brought to zero, as above described, a suitable time delay switch 78 opens the control circuit 70, thus deenergizing solenoid 76 and permitting a spring '79 to open the closing switch 38. Since the control circuit 70 and, the cam 73 acted during the above-described sequence to produce closing of the vac- The precharging circuit for the commutating capacitor 35 may be of any suitable conventional form. It is schematically shown in the drawing as comprising a source having opposed terminals 90 and 91 and a switch 92 that can be closed to connect the commutating capacitor across the terminals 90 and 91. Upon closing of the switch 92, the capacitor is charged through a current limiting resistor 94 to a voltage that will be determined by the voltage of the source 90, 91 and with the polarity shown in FIG. 1.

In the embodiments of FIGS. 1 and 2, I use a movable electrode device 38 as the circuit closing means for the quenching circuit 36. Another way of closing this quenching circuit 36 at the desired instant is shown in FIG. 3, where a triggered vacuum gap device 138 is used instead of the switch 38 of FIGS. 1 and 2. This triggered vacuum gap device is preferably of the design disclosed and claimed in Patent No. 3,087,092, Latferty, assigned to the assignee of the present invention. As such, it comprises a pair of spaced apart main electrodes 140 and 142 disposed in a highly evacuated chamber 144 and defining a main gap 145 therebetween. Disposed adjacent the main electrode 142 is a trigger electrode 146 defined by a hydrogen-impregnated titanium film on a ceramic supporting rod 148. This ceramic supporting rod 148 is disposed coaxially of the main electrode 142 and is suitably sealed to the main electrode 142 about its outer periphery. A portion of the ceramic supporting rod 148 is uncoated and defines a trigger gap along this uncoated surface that electrically isolates the trigger electrode 146 from the main electrode 142 under normal conditions. A conductive connection 149 extends through the ceramic rod 148 and across its upper end surface to the trigger electrode 146.

When an electric pulse is applied between the trigger electrode 146 and the main electrode 142, the trigger gap breaks down and the resultant spank liberates a small quantity of hydrogen from the hydrogen-impregnated trigger electrode 146. This hydrogen is quickly ionized and projected into the main gap 145, thus lowering its dielectric strength and initiating a breakdown of the main gap. When the main gap 145 thus breaks down, the commutating capacitor 35 can discharge through the quenching circuit 36 in the manner described hereinabove to force the current through the main interrupting device 2 to zero.

When the current in the quenching circuit 36 finally reaches zero, as above described, a high dielectric strength is automatically reestablished across the main gap 145 of the triggered gap device, and the gap device 138 is thus restored to its original condition. Because the main gap 145 is a vacuum gap, it has a high dielectric strength capable of withstanding the transient recovery voltage appearing across it when the current through the quenching circuit 36 reaches zero.

The above described pulse across the trigger gap is derived from any suitable conventional pulse source such as schematically shown at 150 connected in a pulse circuit 149, 151 that extends between the trigger electrode and the main electrode. This pulse source 150 is rendered operativeby a suitable switching device 1 52 which operates to complete the pulse circuit 14 9, 151 in response to completion of control circuit 70 when the movable contact 28 of the main interrupter 25 reaches a predetermined point in its opening stroke. The control circuit 70 is completed by the components 72 and 73 in the same general manner as disclosed hereinabove with respect to FIG. 1.

An advantage of using the triggered vacuum gap device of FIG. 3 instead of the switch 38 of FIGS. 1 and 2 is that the gap device can close the quenching circuit 36 at a more precisely controlled instant since there are no large moving parts to be operated as a prior condition to such circuit-closing. Another advantage is that the vacuurn gap device automatically reopens the quenching circuit 36 after an interrupting operation and automatically reestablishes a high dielectric strength gap in this circuit. There is thus no need to provide any controls for returning electrodes to a separated position, as there is with the arrangement of FIGS. 1 and 2 when it is to be reset to its original condition.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Means for interrupting a high voltage D.-C. circuit that comprises a smoothing reactor connected in series with a load across a source of direct current, comprising:

(a) a circuit interrupter adapted to be electrically lo-' cated between said reactor and said load and comprising relatively movable contacts for connection in series wtih said reactor and said load,

(b) means for normally maintaining said contacts in engagement to enable load current to flow therethrough,

(c) arc-initiating means for separating said contacts to draw an arc therebetween,

(d) a normally-open arc-quenching circuit connected across said contacts and comprising normally-open circuit-making means and a commutating capacitor connected in series circuit relationship with each other,

(e) means for precharging said commutating capacitor,

(f) means for forcing the current through said are to zero comprising means responsive to operation of said arc-initiating for closing said circuit-making means upon initiating of said are to discharge said commutating capacitor through said quenching circuit and through said arc in a direction opposite to the direction of load current therethrough,

(g) and means for limiting the magnitude and frequency of the recovery voltage appearing across said circuit interrupter when the current therethrough is forced to zero comprising the series combination ofa normally-open gap device and a resistor connected across said smoothing reactor, and means for causing said gap device to break down and effectively connect said resistor across said reactor immediately after the currentthrough said interrupter reaches zero.

2. The circuit interrupting arrangement of claim 1 in which said gap device is broken down by the voltage developed thereacross by said recovery voltage.

3. The circuit interrupting arrangement of claim 1 in which said circuit interrupter comprises a vacuum-type circuit interrupting unit.

4. The circuit interrupting arrangement of claim 1 in which said resistor has a resistance in ohms between where L is the inductance of said smoothing reactor in 9 16 henries and C is the capacitance of said commutating References Cited by the Examiner capacitor in farads- UNITED STATES PATENTS 5. The circuit interrupting arrangement of claim 1 in 1 755 111 4/1930 Gay 323 76 X Which said circuit-making means comprises a triggered 1:792:340 2/1931 twenmag. n vacuum gap device that includes a trigger gap, said means 5 2 49 59 3 195 Kesselring 3 7 X for closing said circuit-making means comprising means for sparking over said trigger gap to render said vacuum SAMUEL BERNSTEIN Pr'mary Examiner gap device conductive. R. V. LUPO, Assistant Examiner. 

1. MEANS FOR INTERRUPTING A HIGH VOLTAGE D.-C. CIRCUIT THAT COMPRISES A SMOOTHING REACTOR CONNECTED IN SERIES WITH A LOAD ACROSS A SOURCE OF DIRECT CURRENT, COMPRISING: (A) A CIRCUIT INTERRUPTER ADAPTED TO BE ELECTRICALLY LOCATED BETWEEN SAID REACTOR AND SAID LOAD AND COMPRISING RELATIVELY MOVABLE CONTACTS FOR CONNECTION IN SERIES WITH SAID REACTOR AND SAID LOAD, (B) MEANS FOR NORMALLY MAINTAINING SAID CONTACTS IN ENGAGEMENT TO ENABLE LOAD CURRENT TO FLOW THERETHROUGH, (C) ARC-INITIATING MEANS FOR SEPARATING SAID CONTACTS TO DRAW AN ARC THEREBETWEEN, (D) A NORMALLY-OPEN ARC-QUENCHING CIRCUIT CONNECTED ACROSS SAID CONTACTS AND COMPRISING NORMALLY-OPEN CIRCUIT-MAKING MEANS AND A COMMUTATING CAPACITOR CONNECTED IN SERIES CIRCUIT RELATIONSHIP WITH EACH OTHER, (E) MEANS FOR PRECHARGING SAID COMMUTATING CAPACITOR, (F) MEANS FOR FORCING THE CURRENT THROUGH SAID ARC TO ZERO COMPRISING MEANS RESPONSIVE TO OPERATION OF SAID ARC-INITIATING FOR CLOSING SAID CIRCUIT-MAKING MEANS UPON INITIATING OF SAID ARC TO DISCHARGE SAID COMMUTATING CAPACITOR THROUGH SAID SEQUENCHING CIRCIUT AND THROUGH SAID ARC IN A DIRECTION OPPOSITE TO THE DIRECTION OF LOAD CURRENT THERETHROUGH, (G) AND MEANS FOR LIMITING THE MAGNITUDE AND FREQUENCY OF THE RECOVERY VOLTAGE APPEARING ACROSS SAID CIRCUIT INTERRUPTER WHEN THE CURRENT THERETHROUGH IS FORCED TO ZERO COMPRISING THE SERIES COMBINATION OF A NORMALLY-OPEN GAP DEVICE AND A RESISTOR CONNECTED ACROSS SAID SMOOTHING REACTOR, AND MEANS FOR CAUSING SAID GAP DEVICE TO BREAK DOWN AND EFFECTIVELY CONNECT SAID RESISTOR ACROSS SAID REACTOR IMMEDIATELY AFTER THE CURRENT THROUGH SAID INTERRUPTER REACHES ZERO. 